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A detailed description of each task can be found in the SAS documentation, both in HTML and Postscript format. A step by step description of the image mode chain and examples of the processing by individual execution of all tasks is given in the SAS User's Guide and also at this location.
OM image data are fully processed by the XMM-SAS Pipeline: for each exposure of a given observation all necessary corrections are applied to the data files. Then a source detection algorithm is used to identify the sources present in the image. Standard aperture photometry is applied to obtain the count rates for all detected sources. These rates are corrected for coincidence losses and dead time of the detector and finally OM instrumental magnitudes and standard colour corrections are computed. A final source list is obtained from all exposures and filters. Astrometric corrections are applied to each source's position and also the whole image is converted to sky co-ordinates space and rotated so as to have the North on the top. Default image windows are also combined to obtain a mosaic (per filter) of the FOV.
In principle there is no need for further data reduction. The observation source list should contain the calibrated data with their errors. However, some checking is convenient to verify the consistency of the data output.
The whole data processing can be repeated easily by any Guest Observer or Archives User, should any calibration file be updated, and what is more important, in case of doubtful results: the pipeline applies default options in all SAS tasks which eventually can be changed by the GO in order to improve the quality of the results. In particular, the source detection is very sensitive to the artifacts which are very common in OM.
When comparing the data files obtained from the standard SSC pipeline with those obtained by running omichain, the user will notice two differences, the PPS files are compressed while the products from omichain are not, and in addition some intermediate files, with name starting with I, are preserved.
We outline here the checking that any user should perform on OM processed data (by the standard Pipeline or by running SAS) and the use of one of the tasks, omdetect, where the user can modify parameters affecting the source detection and therefore the overall results of the data analysis.
implot set=P0125320701OMS002SIMAGE2000.FTZ withsrclisttab=yes srclisttab=P0125320701OMS002SWSRLI2000.FTZ device='/XW'
This will overplot the detected sources on the corresponding
and it will allow us to check that all sources are real and that the
we are interested in have been properly identified.
If the background is strongly affected by straylight features, this check is very important
and then loading the corresponding region file I0125320701OMS002REGION0000.ASC (this file is not a PPS product).